Thank you for your question.
The largest project we currently know has about 20.000 users in the system and about 80.000 news articles. The users implementation is inherited directly by Asp.Net, and there is no upper limit. To the question about the number of the online users at the same time - you should really have several servers in load-balanced environment, and yes, Sitefinity depends on the database server hardware. We know a case with several thousands users online at the same time, with two front end servers and one database server, but the caching is enabled on the content items.
This is pretty interesting scenario for us. Please, do let us know if you find something, or reach higher limits.
the Telerik team